BOSTON — A man in Orlando, Fla., who was being interviewed early Wednesday morning by law enforcement officers about his ties to the deceased Boston Marathon bombing suspect was fatally shot after he tried to attack the officers with a knife, according to a federal law enforcement official.

The officers — who included an FBI agent from the Boston field office and two Massachusetts state troopers — were questioning the man, Ibragim Todashev, about whether he had played a role in a triple murder on Sept. 11, 2011, in Waltham, Mass., which had been one of the biggest mysteries in the aftermath of the bombings.

The officers had been interviewing Mr. Todashev in his apartment for some time when he tried to attack them, the official said.

“The investigators were working on the theory that he and Tamerlan,” had played a role in the murder, said the official, referring to Tamerlan Tsarnaev, the deceased marathon bombing suspect. One of the victims was a friend of Mr. Tsarnaev.

The FBI agent received minor injuries that required stitches, the official said. The official said that the authorities had spoken to Mr. Todashev at least twice since the April 15 bombings, which killed three people and injured about 200.

The murders in Waltham were considered one of the more violent acts in the Boston area in recent years. Three men were found with their throats slit and sprinkled with marijuana.

The authorities believe that Mr. Todashev and Mr. Tsarnaev were involved in the murders and are seeking to determine whether the police missed an opportunity to thwart the marathon attacks. They have not ruled out that Mr. Tsarnaev's younger brother, Dzhokhar, played a role in the murders.

The mother of the Boston Marathon bombing suspects said in a telephone interview that her sons knew Mr. Todashev.

The mother, Zubeidat Tsarnaeva, said Tamerlan Tsarnaev and Mr. Todashev saw each other regularly in Boston, though they were not particularly close, and that Ibragim had moved to Florida around two years ago.

“Tamerlan said he was a good guy, he said he was a boxer or something,” she said in a telephone interview from Dagestan. She said she had broken down when she heard the news on Wednesday.

“Now another boy has left this life,” she said. “Why are they killing these children without any trial or investigation?”

Since the attacks, the FBI and state and local law enforcement officials in Boston have sought to interview friends and others who knew the Tsarnaev brothers.

Investigators want to know how the brothers were radicalized, and they want to determine if there were accomplices.

The FBI has also focused on Chechens who may have ties to extremists in Russia. Before the attacks, the bureau had not thought that they were a significant threat in the United States.

As part of those efforts, the FBI has questioned many members of the small community of ethnic Chechens in the United States. Since April 29, agents have repeatedly interviewed a Chechen refugee and former rebel fighter, Musa Khadzhimuratov, of Manchester, N.H. Tamerlan Tsarnaev used a firing range in Manchester to practice shooting and bought fireworks in New Hampshire to extract the explosive powder used in the marathon bombs.

Mr. Khadzhimuratov, 36, and his wife, Madina, 32, say they had only brief social visits with Tamerlan Tsarnaev, including one a few weeks before the bombing. They said they did not know about his purchase of fireworks or guns and had no hint that he was plotting the Boston attack.

Some advocates for the Chechen community have expressed concern that Russian intelligence officers might be steering the FBI to target Chechens in the United States who are hostile to Russia but have nothing to do with terrorism.